Announce snippet search deprecation

Proposal

Since snippets are no longer being actively supported, we should remove snippet search as a clean up and to remove unnecessary data from Elasticsearch.

Add a deprecation notice for removing snippet search (before %18.0). Follow the deprecation guidelines:

• Deprecations should be announced on the Deprecated feature removal schedule.
• Deprecations should be announced no later than the third milestone preceding intended removal: if we want to remove in %18.0, announce deprecation before %17.7.

Feature removal will be done seprately in Remove snippet search (#480316)